1. Field of the Invention:
The present invention relates to a numerically controlled machine tool capable of machining a noncircular workpiece such as a cam shaft.
2. Description of the Prior Art:
Generally, in a numerically controlled grinding machine for grinding a non-circular workpiece, the feed of the grinding wheel perpendicular to a spindle axis is controlled according to profile data and machining cycle data.
The profile data comprises the amount of movement of the grinding wheel per unit angle rotation of the spindle which defines the profile generating movement of the grinding wheel along the finished shape of the workpiece. On the other hand, the machining cycle data comprises a numerical control program to control a machining cycle which includes rapid feed, rut-in feed and retracting feed of the grinding wheel.
In the numerically controlled grinding machine tool of the aforementioned type, the ability of the main spindle and the grinding wheel feed axis to accurately follow command data are needed so as to grind the workpiece precisely.
In order to improve machining accuracy, the assignee of this invention has proposed a method, which is disclosed in U.S. Patent Application Ser. No. 07/092,689, filed Sep. 3, 1987, now pending. In this method, a machine tool is operated according to ideal profile data which is calculated from the ideal final shape of a workpiece. During operation, the positional changes of the tool and the main spindle are measured in order to obtain actual movement data. After that, position compensation data is calculated by comparing the ideal profile data and the measured movement data, and the ideal profile data is compensated for with the position compensation data in order to obtain execution profile data, which is used in the actual machining operation.
In such method, however, the position compensation data is set at a constant value regardless of the change in the rotational speed of the main spindle. Accordingly, machining errors arise, even if the compensated execution data is used for the actual machining operation.
Namely, when a workpiece is machined without the above-mentioned compensation, a machining error which changes depending upon the rotational speed of the main spindle arises as shown in FIG. 1. Accordingly, the machining error cannot be eliminated with the constant position compensation data in the case that the rotational speed of the main spindle changes during machining operation.